Members online now must have been done with either a template edit or an add-on as the user group colour doesn't display normally.
For "Members past 24 hours", that is an add-on so you will need to ask in the discussion thread for that add-on on how to remove the user group colour.
Perform template searches for rich="true" and {xen:helper richUserName, $user} and comment them out or remove them in the templates that come up, except for where you want user group color displaying.